Diecasting method and device for carrying out the same

ABSTRACT

The invention relates to a diecasting method used to produce cast parts from a semi-solidified alloy melt. According to the inventive method, the alloy melt is transformed to a semi-solidified state by stimulating the crystallization process, it is then introduced into a casting chamber and the cast pieces are produced by applying pressure. First an exogenous metal suspension is produced outside the casting chamber in a closed-off space that functionally communicates with the casting chamber and forms a casting unit therewith. The metal suspension is set into motion before it is introduced into the casting chamber, thereby forming a hollow rotating body that is maintained rotating until the suspension homogeneity for the casting reached. The invention further relates to a device for carrying out the inventive method. The device comprises a vertical casting chamber with a plunger and a treatment container disposed outside the casting chamber. The treatment container is functionally rigidly linked with the casting chamber and forms a casting unit therewith.

DESCRIPTION

[0001] The invention relates to a die casting method for producing cast parts from a semi-solidified alloy melt, whereby the alloy melt is transformed into a semi-solidified state by stimulating the crystallization process, then introduced into a casting chamber, and the cast parts are produced under pressure. Furthermore, the invention relates to a device for carrying out the method.

[0002] It is known that the thermodynamic state of the metal plays a decisive role in the various treatment processes. The effects connected therewith can be exploited not only for facilitating the deformation process, but employed also for substantially influencing the morphology of the resulting crystalline forms and the properties of the cast part. The alloy loses its strength as the temperature rises, which is of basic importance for shaping process, and the capability for plastic deformability is obtained at the same time as a property dependent upon the structure.

[0003] In the thermodynamic transition phase in the liquid-to-solid temperature range, the alloy melt is in the semi-solidified state. Said state is therefore referred to as the “metallic suspension”, which is employed as the preliminary material for the novel die casting method. Now, the invention is dealing with the phenomenological behavior of the crystallizing liquids and their rheological properties, and exploits the same for the production of cast parts from the semi-solidified state. A preset volume of the melt in subjected in this connection to a special treatment before the melt enters a casting chamber, whereby the method is devised in such a way that the pre-metered metal can be changed in the die casting equipment without any loss in quality.

[0004] It has been known from preliminary tests that the properties of the preliminary material can be influenced in the semi-solidified state by various chemico-physical methods and by controlled heating or cooling. However, the goal was now to find out how the preliminary material can be produced as a homogeneous metallic suspension and poured into the die casting equipment without loss in quality.

[0005] A method for producing cast parts from the semi-solidified state of the melt is known from EP 0 841 406 A1. This patent describes a multi-stage process, the objective of which is to produce the parts from the semi-solidified alloys on a vertical die casting machine. The transport of the preliminary material into the casting chamber takes place following a nucleus formation phase in the superheated melt followed by a subsequent granulation process. A vibrator device was employed for that purpose, by which the formation of nuclei in the crystallizing melt is stimulated. In order to obtain in this process the state of suspension, the semi-solidified preliminary material was heated in a controlled manner. After the casting chamber was communicatively linked up with the container for the preliminary material, the latter was pressed into the die casting mold.

[0006] The known method is similar to the thixoforming process and is consequently afflicted with the following drawbacks:

[0007] (1) It is not a process that can be carried out in a closed processing space. The method is therefore not suitable for reducing the gas content in the solidified cast part.

[0008] (2) Increasing weights of the parts and longer solidification intervals pose the risk that segregation may take place in the preliminary material. This can be the cause for undesirable morphological separations, so that the isotropy of the cast parts and their mechanical properties are deteriorated.

[0009] (3) Since the preliminary material continually changes its properties in the suspended state as it is being cooled or heated, the filling process is decisive for the quality of the cast. In the known process, eddies develop in the freely dropping stream of metal as it is being poured into the casting mold, which distinctly reduces the quality of the cast.

[0010] EP 0 733 421 A1 discloses another casting process in which the casting chamber is filled with the help of a casting plunger after the predetermined volume of the melt has been metered in. The preset volume of the melt is transformed into a state of suspension by additional cooling and mixing in the electromagnetic field, and subsequently pressed into the die casting mold.

[0011] It is in fact possible with the known method to enhance the quality of the volume of the melt before it is poured; however, it is not possible to maintain the desired quality at a constant level over a longer period of time. The fact is that it has been found that coarse structural separations occur, and that, in connection therewith, lower values of mechanical properties are obtained as a result of changes occurring in the conditions of crystallization in the melt.

[0012] Now, the inventors addressed the problem of achieving an enhanced homogeneity of the structure as compared to the products obtained in the prior art, whereby the amount of resulting column-shaped crystals or transcrystallites and the amount of coaxial or globular crystals were used as the quality yardstick. The experimental results showed that the enhanced isotropy of the cast parts can be demonstrated by the type of crystalline shapes obtained, and that particularly high strength values and a very good tenacity behavior can be proven in this way.

[0013] In connection with the method known heretofore, a substantial source for defects lies in that for producing the suspension, the melt was subjected outside of the casting chamber to a structural change resulting from electromagnetic stirring, whereby the desired structure was dependent upon the crystallization process taking place under the influence of the electromagnetic field. As soon as the liquid metal is pressed by the centrifugal forces from the center of the chamber to the walls, high forces of acceleration occur that cause the formation of a liquid cylinder- or cone-shaped jacket of the melt in the casting chamber. It is not possible to uniformly fill the die casting mold with such a preliminary material, so that it was not possible at that time to produce homogeneous cast bodies with isotropic properties. The only possibility that remained available was to reduce the field strength, which then, however, led to quality loss and then to a structural anisotropy of the preliminary material, which both were responsible for the low mechanical properties of the cast part.

[0014] The goal of the present invention is a die casting method by which a homogeneous state of the suspension of the preliminary material is reliably assured. Said preliminary material is then filled under pressure into a pressure chamber while it is in said state of suspension. A further goal consists in that the preliminary material is produced from an exogenous metallic suspension. In connection with the die casting method developed by the present invention, the goal is to develop a continuous casting process, whereby all stages of the method can take place in a closed casting system. It is possible with the novel die casting method to produce from the metallic suspension cast parts exhibiting an excellent quality.

[0015] The aforementioned problems are solved according to the invention with a method of the type specified above in that the preset volume of the melt is put into rotation outside of the casting chamber in a closed processing container, and a homogeneous metallic suspension of the exogenous type is produced in this process, from which a cast part is then produced after the suspension has been filled into the casting chamber. The casting chamber remains in the communicatively linked-up condition throughout the duration of the method and the entire method, which is comprised of individual steps, is devised in the form of a closed casting system.

[0016] So as to realize the production method in a manner as specified above, provision is made in the course of the filling process to first fill the treatment container with the predetermined volume of the melt, to then produce a homogeneous metallic suspension in the exogenous way, and to then transport said suspension into the casting chamber by means of a special transport chamber, for which purpose provision is made for a vertically arranged casting chamber with a filling opening.

[0017] A special feature of distinction of the method as defined by the invention is the way in which the metallic suspension is prepared. As compared to the methods mentioned above, in which the preset volume of the melt is transformed into the suspension state by external cooling, which provides the suspension with an endogenous nature, a basically different approach has been selected in connection with the present invention in that an exogenous suspension is produced abruptly. The core of such an approach is the rapid and uniform cooling of the melt, which is possible only by introducing additional micro-heat processes into the preset volume of the melt. A powder produced from the same alloy as the volume of the melt is used as coolant in order to be able to obtain the required cooling effect in the shortest possible time. The cooling effect lies in a reduction of the excess superheating temperature, whereby the following two crystallization processes, which take place in parallel, occur:

[0018] The melt is cooled from the interior up to the preset temperature, or at least to the temperature of the liquid state, and crystallization nuclei are developed by the additional cooling. Said crystallization nuclei determine the morphology of the structure. Their uniform distribution in the forming metallic suspension is of primary importance in order to assure that the same conditions of crystallization exist throughout the volume of the suspension.

[0019] In said process, the cooling powder is introduced into the melt under pressure, for which purpose air, argon or nitrogen are employed as carrier gas.

[0020] A special design of the treatment container is required for producing metallic suspension following conditioning. According to the invention, such a treatment container is a closed space whose outlet opening can be sealed with a stopper bar. The treatment container is placed in an electromagnetic field and the melt filling is set in motion in said closed space and maintained there until the conditioning process is completed.

[0021] The motion of the melt leads to the formation of a sheared-off material within the area of the solidification front and the crystallization process is stimulated by the melts of said front in the liquid range. This broadens the range of the state of suspension.

[0022] Such range of suspension is characteristic of the entire material volume in the proposed process. From the outset, the forces acting on the metal melt in the form of the electromagnetic field are required in order to shear off the solidifying material within a narrow solidification range and to set the entire volume of the suspension into a motion at which the suspension retains its capability of flowing over a longer period of time while producing the homogeneity desired for the quality of the cast part.

[0023] A formation of hollow, partially liquid rotating bodies takes place in the course of said process. In such a process, the suspension flows to the walls of the treatment container and is maintained in the state of motion until the required casting temperature and casting homogeneity have been reached. It has been found on an experimental basis that the homogeneity of the suspension is primarily dependent upon the value of the so-called “coefficient of gravitation (K)” that emerges on the free surface of the hollow, partly liquid rotating body. Its minimum limit was determined, and based on the quality analysis it was found that the desired morphological isotropy can not be reached with a reduction of the coefficient in which “K” amounts to less than 10.

[0024] The inventors were successful also in obtaining an experimental equation according to which the homogeneity measure of metallic suspensions is determined by a coefficient of homogeneity (X) whose lower efficient limit for aluminum alloys comes to the value 3.8×10⁸ A²/s.

[0025] The coefficient of homogeneity can be determined based on the following relation:

X=N×H ² ×R ²

[0026] wherein

[0027] N=the number of rotation on the free surface of the partly liquid, rotating body;

[0028] H=the strength of the magnetic field; and

[0029] R=the average wall thickness of the partly liquid, rotating body.

[0030] The special design of the treatment container permits converting the preset volume of the melt into a metallic suspension under constant technological conditions. Such a suspension exits from the treatment container as a homogeneous suspension material by way of the outlet opening and then flows into the casting chamber.

[0031] A particularly preferred embodiment of the invention is characterized by a transport chamber specially provided for transporting the suspension. Said transport chamber communicatively connects the treatment container and the casting chamber; it is equipped with a transport plunger, and it can be mounted on the vertically arranged casting chamber either at a right or at an acute angle.

[0032] According to the invention, the transport plunger has the two following functions:

[0033] To rapidly admit the metallic suspension or the remainder thereof into the casting chamber and to tightly seal the latter after it has been filled. The plunger is designed for that purpose in such a way that its frontal contour is a continuation of the inner contour of the casting chamber, so that when the casting plunger is moving, no interference occurs within the zone of the filling opening from the side of the transport plunger.

[0034] The operation in which the casting chamber is filled with the metallic suspension is characterized by the so-called “rising filling level”. While the casting chamber is filling with the suspension, the casting plunger is (notably synchronously) displaced downwards.

[0035] The invention is explained in greater detail in the following with the help of an exemplified embodiment shown in the drawing, in which:

[0036]FIG. 1 is a schematic representation of a die casting device as defined by the invention, with which cast parts can be produced from a homogeneous metallic suspension of the exogenous type; and

[0037]FIG. 2 is a cutout “A” of FIG. 1 within the area of the mouth of the transport chamber 6 feeding into the casting chamber 7.

[0038] In the die casting machine shown in FIG. 1, provision is made for a treatment container 1 that is equipped with a stopper bar 2 and a pipeline 3 for admitting cooling powder. Furthermore, by means of a melt line 4, the container is connected with the furnace 5 for keeping the melt hot, and connected with the vertically arranged casting chamber 7 by means of a transport chamber 6.

[0039] Furthermore, the treatment container 1 is inserted in an electromagnetic agitating device 8 which, as an integrating system is comprised of an induction and control unit in the form of a closed component of the die casting machine. The diagram shows that the transport chamber 6 is mounted on the casting chamber 7 at an acute angle. However, it is possible also to link up the structural elements at a right angle and to provide them with the transport plunger 9. Such a transport plunger not only frees the transport chamber 8 of metal residues but also seals the filling opening 10 in the casting chamber 7. The casting chamber is aligned in the usual manner with a casting plunger 11.

[0040] An electromagnetic field permitting a stirring motion of the melt is generated in the induction unit with a control system (not shown in the diagram). The melt 12 is subsequently conveyed by way of a melt line 4 from the furnace 5 keeping the melt hot, into the treatment container 1, which is located in the induction coil 8. By applying the rotating magnetic field, the preset volume of the melt is set in motion, notably in a closed space because the outlet opening of the treatment container is tightly sealed by the stopper bar 2. Propelled by centrifugal forces, the melt flows to the walls of the container and forms a liquid, hollow rotating body (according to the drawing, the rotating body has a conical shape). A coefficient of gravitation “K” is generated in this process on the free surface of the rotating body whose value is determined by the velocity with which the different liquid layers are displaced against one another. By advantageously designing the treatment container it is possible already in this stage of the process to set the entire volume of the melt in motion, notably not only at the solidification front. The entire volume of the melt is maintained in such a motion until the desired condition or homogeneity of the suspension have been reached.

[0041] A short time after a hollow, liquid rotating body has formed from the superheated melt, an amount of powder is introduced into the rotating melt via a powder metering device 3 (shown in the diagram as a pipeline) that is adequate for producing a cooling effect. According to the invention, the cooling powder is admitted into the melt under pressure, notably according to a pulsating regimen. The exogenous metallic suspension, which is abruptly produced in the treatment container 1, is a result of the three following merging processes:

[0042] The first process is part of the heat exchange process, in which the powder material extracts the excess heat from the superheated melt. This produces a number of cooled areas of the suspension where the temperature is below the liquid level.

[0043] The second process is connected with the way in which the powder is introduced into the melt. As the process takes place under pressure, the powder particles do not remain on the inner surface of the liquid, rotating bodies, but penetrate the melt very deeply and act as efficient inner heat absorbers, which causes an exogenous metallic suspension to be simultaneously produced throughout the entire volume of the melt. As the powder is introduced into the melt in a pulsating manner, elastic oscillations develop in the liquid metal by which the formation of new nuclei of crystallization in the preset volume of the melt is stimulated even more by a micro-cavitation effect.

[0044] The third process is the constant rotational motion of the preset volume of the melt that takes place in the course of and parallel with the two processes specified above. By devising the method as defined by the invention in an advantageous manner, it is possible to maintain the entire volume of the melt in motion in the closed space. A preliminary material is formed in this process first from the melt and subsequently from the metallic suspension that represents the hollow rotating body. The coefficient of gravitation produced on the free surface of said rotating body is extremely important because it determines the homogeneity of the metallic suspension. The following parameters are influenced:

[0045] (1) The geometry of the rotating body, which secures the stability of adjusted technological parameters.

[0046] (2) Thermal and chemical uniformity.

[0047] (3) No additional absorption of water; viscosity accuracy.

[0048] (4) An acceleration at which the required shearing force is present and uniformly distributed not only at the solidification front, but throughout the entire preset or rotating volume of the melt.

[0049] (5) Mixing, by which the metallic suspension is provided with the desired homogeneity.

[0050] (6) A highly efficient dissipation of heat takes places owing to the close contact between the melt and the powder particles.

[0051] Excessively cooled areas of the metal occurring in the melt are broadened owing to the continuous mixing, and grow together. Since this takes place under identical technological conditions, the suspension is produced simultaneously throughout the entire volume of the melt, so that its homogeneity is optimally adjusted.

[0052] An important consequence of the homogeneity of the suspension so obtained is the formation of roundly shaped crystalline forms that are uniformly distributed in the solidifying cast part, which leads to superior mechanical properties.

[0053] After the homogeneity of the suspension has been reached, the suspension is drained from the treatment container 1 by pulling out the stopper bar 2, and the suspension is received in the transport chamber 6. In this process, the suspension either may be set in motion, or it may directly flow from the treatment container 1, which has no bearing on the quality of the suspension, but an influence on the draining time.

[0054] As the metallic suspension produced in such a manner has a high potential of kinetic energy, it flows very rapidly towards the transport chamber 6 in the direction of the casting chamber 7, which starts to fill by way of the filling opening 10. Simultaneously with the flow of suspension into the casting chamber, the casting plunger 11 is synchronously displaced downwards in order to permit the filling level to rise. So as to be able to protect the transport chamber 6 against the metallic suspension flowing in through the filling opening especially at high acceleration of the plunger, the transport chamber is equipped with the transport plunger 9. After the filling operation has been completed, the transport plunger 9 is pushed forward and closes the filling opening 10 and thereby seals the casting chamber 7. The transport plunger 9 is designed in this connection in such a way that its frontal contour is a continuation of the inner contour of the casting chamber 7. The metallic suspension present in the casting chamber fills the pressure chamber by means of acceleration of the plunger, so that a cast part is produced from the semi-solidified preliminary material.

[0055] First tests with the method as defined by the invention have distinctly shown that the cast aluminum alloy parts produced according to the method as defined by the invention exhibit a finer and isotropic morphology of the structure that is characterized by uniform, roundly shaped, crystalline forms of primary phases. The consequence thereof is an increase in mechanical properties as compared to products of conventional methods. Said values are summarized in table 1 for the alloy AlSi9Cu3 in the cast condition. TABLE 1 Homogeneity Tensile Elongation Elongation coefficient X strength limit at break Method A²/s N/mm² N/mm² % Conventional 1.1 207 126 2.4 Novel method 3.0 214 130 2.5 Novel method 3.8 234 151 2.88 Novel method 5.0 251 157 3.12

[0056] The parts produced according to the novel method exhibited substantial improvements already in the cast condition. A firm link exists between the general increase of mechanical properties and the technological conditions under which the metallic suspension was produced. By applying the concept as defined by the invention it has been made possible to produce cast parts with high-grade mechanical characteristics and a constant quality. 

1. A die casting method for producing cast parts from a semi-solidified alloy melt, whereby the alloy melt is transformed to a semi-solidified state by stimulating the crystallization process, introduced into a casting chamber, and the cast parts are produced under pressure, characterized in that an exogenous metallic suspension is produced outside of the casting chamber in a closed space functionally connected with the casting chamber and forming one casting unit with the latter, whereby the metallic suspension is set in motion before entering the casting chamber, and whereby a hollow rotating body is formed, said body being maintained in the rotating state until the homogeneity of the suspension for pouring it into the chamber has been reached.
 2. The method according to claim 1, characterized in that the stimulation of the crystallization process takes place in a rotating electromagnetic field.
 3. The method according to any one of the preceding claims, characterized in that for stabilizing the rotating melt thermodynamically, a cooling powder is introduced which abruptly puts the melt into a semi-solidified state.
 4. The method according to any one of the preceding claims, characterize in that the formation of the melt takes place under the action of elastic oscillations of the liquid metal.
 5. The method according to any one of the preceding claims, characterized in that the cooling powder is put into a pulsating state and introduced into the melt.
 6. The method according to any one of the preceding claims, characterized in that the cooling powder is introduced into the melt under pressure, whereby air, argon or nitrogen are used as carrier gas.
 7. The method according to any one of the preceding claims, characterized in that the outflowing metallic melt is subjected to the influence of an electromagnetic field.
 8. The method according to any one of the preceding claims, characterized in that the casting plunger is synchronously displaced downwards as the casting chamber is being filled and the rise in the filling level is enforced in this way.
 9. A device for carrying out the method according to any one of the preceding claims, comprising a vertical casting chamber with a casting plunger and a treatment container arranged outside of the casting chamber, characterized in that the treatment container functionally communicates with the casting chamber and forms one casting unit therewith.
 10. The device according to the preceding claim, characterized in that the treatment container is surrounded by a coil generating an electromagnetic field within the treatment container; and that the treatment container is connected with the casting chamber via a transport chamber.
 11. The device according to any one of the preceding claims, characterized in that the treatment container contains a closing device by which the container can be completely sealed during the treatment or during the rotation of the melt.
 12. The device according to any one of the preceding claims, characterized in that the transport chamber is mounted at a right angle in relation to the casting chamber.
 13. The device according to any one of the preceding claims, characterized in that the transport chamber is mounted at an acute angle in relation to the casting chamber.
 14. The device according to any one of the preceding claims, characterized in that the transport chamber is equipped with a transport plunger sealing the filling opening of the casting chamber.
 15. The device according to any one of the preceding claims, characterized in that on its side facing the filling opening of the casting chamber, the transport plunger is designed in such a manner that its frontal contour is a continuation of the inner contour of the casting chamber. 